The present invention concerns equipment for sampling and processing for analysis PAH and other organic compounds, as well as hydrogen fluoride and oxides of sulphur.
PAH stands for polycyclic aromatic hydrocarbons. Many of these compounds are carcinogenic. Other compounds that are desirable to monitor because of their harmful effects include phenols, polychlorinated biphenyls (PCBs), and dioxins. All these compounds occur for example in tar and/or solvents.
It is known that hydrogen fluoride and sulphur oxides are environmental pollutants and health hazards.
The above-mentioned compounds occur, for example, to a greater or lesser extent, in and around anode factories, in raw gas from electrolytic furnaces, in electrolysis halls, in and around incineration plants, in solvents and in places of work where chemicals containing such compounds are used or formed.
Hitherto it has been difficult to monitor emissions of these compounds because the available measuring equipment has not been adequate. Present-day knowledge of the harmful environmental effects of such emissions, even in small quantities, has made it essential to conduct research on measuring equipment and analytical methods which can determine emissions of these compounds with sufficient accuracy.
Besides the desirability of measuring whether these compounds are emitted into the air, and if so in what quantities, it is also desirable in anode factories to be able to draw up material balance reports in the purification plants in order to survey the tar/PAH flow in the factories, and to measure the effect of the purification plants. The material which exists today to carry out such measurements in the anode factories does not provide reliable results, and there is a need for better solutions.
In 1986 Alcoa published its own method 4090A, among other things, measuring particles and hydrocarbon emission from anode factories, including gas to and from purification plants, in J. H. Walker, J. E. Gibb & J. N. Peace, "Sampling and analytical methods for measuring carbon plant emissions", Light Metals, pp. 955-972. This method is a modification of the EPA's reference method 5. The modification involves sampling through probe tubes and filters heated to approximately the temperature in the gas duct. The gas is then cooled and conducted through c. 20 g XAD adsorbent at 20.degree. C. At this temperature the water vapour will condense. After adsorption, therefore, are a condensate trap (empty impinger) followed by two impingers with de-ionized water and one impinger with a drying agent (silica gel). All the impingers are in ice baths. After sampling, all the units are extracted, except the impinger with the drying agent, with methylene chloride. Alcoa's method is complicated because the sampling equipment consists of a relatively large number of collection units in series. This means that the processing and analysis of the samples becomes labour-intensive and has a high risk of error.
In a considerably simpler sampling method used by Hydro Aluminium, the gas sample is extracted through a pipe filled with glass wool (the "Lurgi method"). The gas is then conducted through two absorption flasks with toluene and one empty flask, and straight on into a pump with a gas clock. The flasks are not cooled, and the evaporation loss of the toluene is therefore high. After sampling, the glass wool is boiled out with toluene, filtered and evaporated. The absorption sample in the flasks is also filtered and evaporated. The particles in the solution are extracted with hot toluene, which is evaporated. The sum of the evaporation residues is specified as the total tar quantity. Tar contains a large number of components ranging from heavy to volatile compounds. The consumption of toluene with this method is 600-1000 ml per sampling. The boiling point of toluene is 110.6.degree. C. This leads to great uncertainty in the method, inasmuch as the volatile compounds in tar will evaporate with the toluene. In addition, the evaporation of such quantities of toluene is undesirable, because the toluene vapour is toxic.
European patent application publ. no. 42683 describes an apparatus for capturing gas components in low concentrations. They are captured on various adsorbents which are packed between sinters in an outside pipe. The technique described in the above-mentioned European patent application would not, however, be suitable for determining the tar flow in an anode factory and for similar tasks where the present invention can be used. If one used the apparatus described in EP-42683 for such purposes, problems would arise because the gas volume through the apparatus is too small for isokinetic sampling, and aerosols would not be captured quantitatively. In addition, this technique would entail condensation of water vapour, and it would not be possible to change the adsorbent after sampling.
British patent application publ. no. 2083662 describes an apparatus where fluid is used as an absorption medium to capture gas components. This apparatus could not be used either to determine the tar flow in an anode factory, since the hydrophobic filter would quickly be clogged with tar and the fluid in the pipe would not absorb gaseous tar components quantitatively. Furthermore, the fluid would evaporate or water vapour would condense, depending on the temperature conditions, and this equipment cannot be extracted directly after the sampling.
The same disadvantages from which the technique according to GB-2083662 suffers as regards use in sampling to determine tar content also apply to the apparatus described in German patent document no. 2448001. This German patent application describes an apparatus with specially impregnated filters in series for selective separation of gas components (Cl.sub.2, H.sub.2 S, SO.sub.2) from air.